An optical bench in a wavelength selective switch (WSS) is mounted using a combination of fixed mounts and stress-free mounts. The WSS is packaged in an enclosure including a base, a sidewall, and a lid. The optical switching engine is attached directly to the base. The optical bench is attached to the base and the optical components supported thereon are aligned with the array of switching elements of the switching engine. The optical bench is attached to the base with at plurality of mounts, which include at least one movable mount supporting movement of the optical bench in a plane parallel to the optical bench and at least one fixed mount maintaining optical alignment between the dispersive element and the array of switching elements.